PolyNull and conditional expressions

[wmdietl]

Take this code:

import org.checkerframework.checker.nullness.qual.PolyNull;

class Demo {
    public static @PolyNull String toLowerCaseA(@PolyNull String text) {
        return text == null ? null : text.toLowerCase();
    }
    public static @PolyNull String toLowerCaseB(@PolyNull String text) {
        return text != null ? text.toLowerCase() : null;
    }
}

The two methods perform the same thing, just with flipped logic.
However, for toLowerCaseB we get an error:

Error: [conditional.type.incompatible] incompatible types in conditional expression.
   found   : null (NullType)
   required: @Initialized @NonNull String

[Ao-senXiong]

A small question: why is the type of conditional expression text == null ? null : text.toLowerCase() is @Initialized @PolyNull String at first?